WO2022255354A1

WO2022255354A1 - Magnetism measuring device

Info

Publication number: WO2022255354A1
Application number: PCT/JP2022/022105
Authority: WO
Inventors: 多聞笠島; 秀一大川; 正則小須田; 泰司竹村; バグーストリスナントスコ
Original assignee: Tdk株式会社; 国立大学法人横浜国立大学
Priority date: 2021-06-02
Filing date: 2022-05-31
Publication date: 2022-12-08
Also published as: EP4350377A1; CN117413196A; JP2022185470A

Abstract

[Problem] To reduce the current value and frequency of current flowing in an excitation coil in a type of magnetism measuring device that detects magnetism changes caused by excitation of an object to be measured. [Solution] A magnetism measuring device 1 comprises: an excitation coil C1 that applies an AC excitation magnetic field to an object to be measured that includes a magnetic body P to cause the change in magnetization of the magnetic body P to have a linear response; a detection coil C0 that generates a primary detection signal S1 by detecting a primary AC detection magnetic field generated by the magnetization of the magnetic body P; a magnetic field generation coil C2 that generates a secondary AC detection magnetic field on the basis of the primary detection signal S1; and a magnetism sensor 16 that generates a secondary detection signal S2 including a non-sine wave component by detecting the secondary AC detection magnetic field. Due to the forgoing, it is possible to obtain a secondary detection signal S2 that includes a non-sine wave component without causing the change in magnetization of the magnetic body P to have a linear response, and as a result thereof, the current value and the frequency of the current flowing in the excitation coil can be reduced.

Description

Magnetic measuring device

The present invention relates to a magnetic measurement device, and more particularly to a magnetic measurement device that can be used for magnetic particle imaging.

A magnetic particle imaging device is known as a type of magnetic measurement device that detects magnetization changes caused by exciting a measurement target (see Non-Patent Document 1). A magnetic particle imaging apparatus includes an excitation coil that applies an alternating excitation magnetic field to a measurement object containing magnetic particles, and a magnetic sensor that detects an alternating detection magnetic field generated by magnetization change of the excited magnetic particles. Not only the AC detection magnetic field but also the AC excitation magnetic field is applied to the magnetic sensor. can be separated by signal processing.

However, in order to make the magnetization change of magnetic materials such as magnetic particles non-linear, it is necessary to apply a very strong AC excitation magnetic field to the measurement object. Moreover, in order to detect the magnetization change with nonlinear response with high sensitivity, it is necessary to set the frequency of the AC excitation magnetic field to a high frequency of 20 kHz or higher, for example. Therefore, when the object to be measured has a relatively large size, such as a human body, it is necessary to supply an extremely large high-frequency current to the exciting coil, which is not practical.

Therefore, it is an object of the present invention to reduce the current value and frequency of the current flowing through the excitation coil in a magnetic measurement device that detects changes in magnetization caused by exciting an object to be measured.

A magnetic measurement apparatus according to the present invention includes a first coil that linearly responds to changes in magnetization of a magnetic body by applying an AC excitation magnetic field to a measurement object that includes a magnetic body, and a primary AC detection that occurs due to the change in magnetization of the magnetic body. a first magnetic sensor for generating a primary detection signal by detecting a magnetic field; a second coil for generating a secondary AC detection magnetic field based on the primary detection signal; and detecting the secondary AC detection magnetic field. and a second magnetic sensor that generates a secondary detection signal including a non-sinusoidal component by

According to the present invention, the primary detection signal generated by the first magnetic sensor is converted again into a magnetic field, and this magnetic field is detected by the second magnetic sensor. A secondary detection signal containing non-sinusoidal components can be obtained without This makes it possible to detect the magnetization change of the magnetic material with high sensitivity even if the current flowing through the first coil has a small current value and a low frequency.

The magnetic measurement device according to the present invention may further include a third coil that cancels the AC excitation magnetic field applied to the first magnetic sensor. According to this, it becomes possible to reduce the noise component contained in the primary detection signal.

The magnetic measurement device according to the present invention may further include a signal processing circuit for detecting harmonic components of the secondary detection signal. According to this, it becomes possible to remove the noise component contained in the secondary detection signal.

Thus, according to the present invention, it is possible to reduce the current value and frequency of the current flowing through the exciting coil in a magnetic measurement device that detects magnetization changes caused by exciting the measurement object.

FIG. 1 is a schematic diagram for explaining the configuration of a magnetic measurement device 1 according to one embodiment of the present invention. FIG. 2 is a schematic diagram for explaining the magnetization change of the magnetic material P. FIG. FIG. 3 is a circuit diagram of the magnetic sensor 16. As shown in FIG. FIG. 4 is a schematic graph for explaining the magnetization change of the magnetic material P, in which the vertical axis indicates the magnetization M and the horizontal axis indicates the magnetic field H. As shown in FIG. FIG. 5 is a schematic graph for explaining changes in the secondary detection signal S2, in which the vertical axis indicates the voltage V and the horizontal axis indicates the magnetic field H. As shown in FIG. FIG. 6 is a graph showing the waveform of each signal, (a) is the waveform of the AC exciting current i1, (b) is the waveform of the primary detection signal S1, (c) is the waveform of the secondary detection signal S2, (d ) to (h) show the waveforms of the 3rd, 5th, 7th, 9th and 11th harmonics contained in the secondary detection signal S2.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram for explaining the configuration of a magnetic measurement device 1 according to one embodiment of the present invention.

A magnetic measurement apparatus 1 according to the present embodiment is an apparatus for detecting a magnetic body P within a measurement object located in a measurement area A. As shown in FIG. , an excitation circuit 13 connected to an excitation coil C1; The magnetic material P may be nano-sized magnetic nanoparticles. If magnetic nanoparticles are used as the magnetic material P, it is possible to use the human body as the object to be measured.

The magnets 11 and 12 are arranged so that their S or N poles face each other so that the intensity of the gradient DC magnetic field φ in the measurement area A is almost zero. Coils may be used instead of the magnets 11 and 12 . Also, a mechanism for spatially moving the measurement area A may be provided. The excitation circuit 13 is a circuit that supplies an alternating excitation current i1 to the excitation coil C1, and an alternating excitation magnetic field is applied to the measurement area A by this circuit. The waveform of the AC exciting current i1 is a sine wave. As will be described later, the intensity of the AC excitation magnetic field is set to such an intensity that the magnetization change of the magnetic body P positioned in the measurement area A linearly responds. A linear response means that the magnetization of the magnetic body P changes in the non-saturation region. Therefore, the magnetization change is not limited to being completely linear, and may include some nonlinear components as long as the magnetization change is in the non-saturation region.

FIG. 2 is a schematic diagram for explaining the magnetization change of the magnetic material P. FIG.

In the example shown in FIG. 2(a), when an exciting magnetic field is applied to the magnetic body P, the magnetic body P itself having magnetization M in a predetermined direction rotates, and as a result, the direction of magnetization M changes. Further, in the example shown in FIG. 2B, when an exciting magnetic field is applied to the magnetic body P, the magnetization M inside the magnetic body P rotates. In any of these cases, the direction of the magnetization M of the magnetic body P is changed by applying an exciting magnetic field to the magnetic body P. FIG. Moreover, both the rotation of the magnetic body P itself shown in FIG. 2A and the rotation of the magnetization M inside the magnetic body P shown in FIG. In this embodiment, the change in the direction of magnetization M caused by applying an exciting magnetic field to the magnetic material P is defined as "magnetization change".

The magnetization change of the magnetic material P generates a primary AC detection magnetic field. The primary AC detection magnetic field is detected by a detection coil C0, which is a first magnetic sensor, to generate a primary detection signal S1. In this embodiment, the detection coil C0 is used as the magnetic sensor for detecting the primary AC detection magnetic field, but the magnetic sensor for detecting the primary AC detection magnetic field is not limited to this. It may be the magnetic sensor used. The AC excitation magnetic field is also applied to the magnetic material P existing outside the measurement area A. In the area outside the measurement area A, the direction of the magnetization M is fixed by the gradient DC magnetic field φ having a predetermined intensity. Therefore, substantially no magnetization change occurs. Therefore, the detection coil C0 can selectively detect the magnetization change of the magnetic body P positioned in the measurement area A. FIG.

Also, the AC excitation magnetic field applied to the detection coil C0 is canceled by the cancel coil C3. A canceling current i3 flows through the canceling coil C3 by the compensating circuit 14, thereby canceling out the AC excitation magnetic field applied to the detecting coil C0. However, it is difficult to completely cancel the AC excitation magnetic field applied to the detection coil C0, and some noise components caused by the AC excitation magnetic field are superimposed on the primary detection signal S1.

The primary detection signal S1 is input to the amplifier circuit 15. The amplifier circuit 15 is an analog circuit including a differential amplifier, a filter circuit, and the like, and supplies an AC detection current i2 to the magnetic field generating coil C2 based on the primary detection signal S1. As a result, a secondary AC detection magnetic field is generated from the magnetic field generating coil C2. Here, since the amplifier circuit 15 is an analog circuit, almost no delay occurs, and the secondary AC detection magnetic field is generated substantially in real time according to the primary AC detection magnetic field. The secondary AC detection magnetic field is detected by the second magnetic sensor 16 to generate a secondary detection signal S2.

FIG. 3 is a circuit diagram of the magnetic sensor 16. FIG.

As shown in FIG. 3, the magnetic sensor 16 is composed of magneto-sensitive elements 21 to 24 connected in full bridge connection. The magneto-sensitive elements 21 to 24 include magnetoresistive elements such as TMR (tunnel magnetoresistive effect) elements, GMR (giant magnetoresistive effect) elements, and AMR (anisotropic magnetoresistive effect) elements, Hall elements, MI ( An element such as a magneto-impedance element that has high sensitivity even at low frequencies and is magnetically saturated can be used. The magnetic sensor 16 is configured such that the secondary AC detection magnetic fields generated by the magnetic field generating coil C2 are applied to the magneto-

sensitive elements

21 and 22 and the magneto-

sensitive elements

23 and 24 in opposite directions. As a result, the magnetic sensor 16 outputs a secondary detection signal S2 corresponding to the secondary AC detection magnetic field. The magnetic sensor 16 is not limited to one in which four magneto-sensitive elements are connected in a full bridge, but may be one in which two magneto-sensitive elements are half-bridge connected or one using a single magneto-sensitive element. .

The secondary detection signal S2 is supplied to the signal processing circuit 18 via the amplifier 17. The signal processing circuit 18 generates a tertiary detection signal S3 by extracting harmonic components contained in the secondary detection signal S2. The tertiary detection signal S3 is the final output signal of the magnetic measurement device 1 according to this embodiment, and indicates the magnetization change of the magnetic material P located in the measurement area A. By using an imaging device for imaging the tertiary detection signal S3, it is possible to configure a magnetic particle imaging device.

The above is the configuration of the magnetic measurement device 1 according to this embodiment. Next, the operation of the magnetic measurement device 1 according to this embodiment will be described.

First, the excitation circuit 13 causes an AC excitation current i1 to flow through the excitation coil C1 so that the magnetization change of the magnetic body P located in the measurement area A linearly responds. In other words, the AC excitation current i1 supplied to the excitation coil C1 is set to a current amount sufficiently smaller than the current amount required to cause the magnetization change of the magnetic body P to respond non-linearly.

FIG. 4 is a schematic graph for explaining the magnetization change of the magnetic material P, in which the vertical axis indicates the magnetization M and the horizontal axis indicates the magnetic field H.

As shown in FIG. 4, when the amplitude of the magnetic field H is set to H1, the magnetization M of the magnetic body P is saturated and the magnetization M of the magnetic body P responds nonlinearly between magnetization m1 and magnetization m2. In this case, the detection signal component included in the primary detection signal S1 becomes a non-sinusoidal wave. Here, when the object to be measured is the size of a human body, a strong magnetic field of about 6 mT is required in order to cause the magnetization M of the magnetic material P made of magnetic nanoparticles to respond nonlinearly. On the other hand, when the amplitude of the magnetic field H is set to H2 (<H1), the magnetization M of the magnetic material P changes in the non-saturation region, so linear response occurs between magnetization m3 and magnetization m4. In this case, the detection signal component included in the primary detection signal S1 becomes a sine wave.

In this way, the amount of the AC exciting current i1 is suppressed to a current amount that causes the magnetization change of the magnetic material P to respond linearly. The amount of current is greatly reduced. Here, when the object to be measured is the size of a human body, a magnetic field of 0.1 mT, for example, is sufficient for causing the magnetization M of the magnetic material P made of magnetic nanoparticles to linearly respond. That is, the amount of current is 1/10 or less of that in the case where the magnetization change of the magnetic body P is caused to respond non-linearly.

As described above, in the present embodiment, since the magnetization change of the magnetic body P is caused to linearly respond by the excitation coil C1, the primary AC detection magnetic field of the primary detection signal S1 generated by the detection coil C0 is The resulting detection signal component is a sine wave. The primary detection signal S1 also contains a noise component caused by an AC excitation magnetic field that has not been completely canceled. However, since the noise component is sufficiently suppressed by the cancel coil C3, its level is sufficiently small and the detection signal component is dominant. The primary detection signal S1 is converted into an AC detection current i2 by the amplifier circuit 15, thereby generating a secondary AC detection magnetic field from the magnetic field generating coil C2. The secondary AC detection magnetic field is detected by the magnetic sensor 16 to generate a secondary detection signal S2.

FIG. 5 is a schematic graph for explaining changes in the secondary detection signal S2, in which the vertical axis indicates the voltage V and the horizontal axis indicates the magnetic field H.

As shown in FIG. 5, the amplitude of the detection signal component included in the secondary AC detection magnetic field is H3. The magnetoresistive effect of the magneto-sensitive elements 21 to 24 is saturated for the component of the secondary AC detection magnetic field whose amplitude is H3, and the voltage V of the secondary detection signal S2 is nonlinear between the voltage v1 and the voltage v2. respond. In this case, the detection signal component included in the secondary detection signal S2 becomes a non-sinusoidal wave. On the other hand, the amplitude of the noise component contained in the secondary AC detection magnetic field is H4 (<H3). Since the magneto-sensitive elements 21 to 24 operate in the non-saturation region for the component of the secondary AC detection magnetic field whose amplitude is H4, the voltage V of the secondary detection signal S2 is between the voltage v3 and the voltage v4. linear response.

Here, in order to cause the magneto-sensitive elements 21 to 24 to respond nonlinearly by the detection signal component and to make the magneto-sensitive elements 21 to 24 linearly respond by the noise component, a state in which the magnetic body P does not exist in the measurement area A, that is, the detection Preliminary magnetic measurement operation is performed in a state where no signal component is included, and the gain and filter characteristics of the amplifier circuit 15 are adjusted so that the magneto-sensitive elements 21 to 24 linearly respond to the noise component caused by the AC excitation magnetic field. Good luck.

As a result, the detection signal components contained in the secondary AC detection magnetic field are converted into non-sinusoidal components of the secondary detection signal S2, and the noise components contained in the secondary AC detection magnetic field are converted into sine wave components of the secondary detection signal S2. converted to components. That is, the detection signal component and the noise component included in the primary detection signal S1 are both sinusoidal waves, but are converted into magnetic fields again using the magnetic field generating coil C2, and are then detected secondarily using the magnetic sensor 16. Reconversion to signal S2 separates the detected signal component and the noise component into non-sinusoidal and sinusoidal components.

The secondary detection signal S2 generated in this way is supplied to the signal processing circuit 18 via the amplifier 17. The signal processing circuit 18 generates a tertiary detection signal S3 by extracting harmonic components contained in the secondary detection signal S2. As described above, since the detection signal components included in the secondary detection signal S2 are non-sinusoidal components, harmonics are generated. On the other hand, since the noise component contained in the secondary detection signal S2 consists of sine wave components, almost no harmonics are generated. Therefore, by detecting the harmonic component contained in the secondary detection signal S2, it is possible to selectively extract the detection signal component.

FIG. 6 is a graph showing the waveform of each signal, (a) is the waveform of the AC exciting current i1, (b) is the waveform of the primary detection signal S1, (c) is the waveform of the secondary detection signal S2, (d ) to (h) show the waveforms of the 3rd, 5th, 7th, 9th and 11th harmonics contained in the secondary detection signal S2. In addition, in FIGS. 6B to 6H, solid lines indicate detection signal components, and broken lines indicate noise components.

As shown in FIG. 6(a), the AC exciting current i1 is a sine wave. In the present embodiment, since the magnetization M of the magnetic material P is caused to linearly respond by the AC excitation magnetic field, as shown in FIG. Both components are sine waves. However, in the present embodiment, the magnetic field generating coil C2 and the magnetic sensor 16 are used to change the detection signal component to a non-sinusoidal wave, so as shown in FIG. 6(c), the secondary detection signal S2 The detection signal component contained in is a non-sinusoidal wave, and the noise component contained in the secondary detection signal S2 is a sine wave. As a result, as shown in FIGS. 6(d) to 6(h), large harmonics appear in the detection signal component, while almost no harmonics appear in the noise component. As an example, when the ratio (SN ratio) of the detection signal component and the noise component included in the primary detection signal S1 is 9.6 dB, the 3rd harmonic, 5th harmonic, 7 The signal-to-noise ratios of the harmonic, ninth, and eleventh harmonics are 11.1 dB, 15.3 dB, 14.7 dB, 13.3 dB, and 9.0 dB, respectively.

Therefore, if a predetermined harmonic component is extracted from the secondary detection signal S2 by the signal processing circuit 18, it is possible to extract the detection signal component caused by the magnetization change of the magnetic material P. The detection signal component thus extracted is output to the outside as a tertiary detection signal S3.

As described above, the magnetic measurement device 1 according to the present embodiment causes the magnetization M of the magnetic body P to linearly respond to an alternating excitation magnetic field, while using the magnetic field generating coil C2 and the magnetic sensor 16 to selectively detect the detection signal component. Since it is changed to a non-sinusoidal wave, it is possible not only to greatly reduce the current amount of the AC exciting current i1, but also to reduce the frequency of the AC exciting current i1 to about 10 kHz. can be ensured. Moreover, since the physical device is used to convert the primary detection signal S1 into the secondary detection signal S2, there is no delay unlike in the case of direct signal processing of the primary detection signal S1. This enables magnetic particle imaging of a relatively large measurement object such as the human body.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. Needless to say, it is included within the scope.

1 magnetic measurement device 11, 12 magnet 13 excitation circuit 14 compensation circuit 15 amplifier circuit 16 magnetic sensor 17 amplifier 18 signal processing circuits 21 to 24 magneto-sensitive element A measurement area C0 detection coil C1 excitation coil C2 magnetic field generation coil C3 cancellation coil P magnetism body S1 primary detection signal S2 secondary detection signal S3 tertiary detection signal i1 AC excitation current i2 AC detection current i3 cancellation current φ gradient DC magnetic field

Claims

a first coil that linearly responds to the magnetization change of the magnetic material by applying an alternating excitation magnetic field to a measurement object that includes the magnetic material;
a first magnetic sensor that generates a primary detection signal by detecting a primary AC detection magnetic field generated by magnetization change of the magnetic material;
a second coil that generates a secondary AC detection magnetic field based on the primary detection signal;
and a second magnetic sensor that generates a secondary detection signal containing a non-sinusoidal wave component by detecting the secondary AC detection magnetic field.
The magnetic measurement device according to claim 1, further comprising a third coil for canceling the alternating excitation magnetic field applied to the first magnetic sensor.
The magnetic measurement device according to claim 1 or 2, further comprising a signal processing circuit for detecting harmonic components of the secondary detection signal.